Reflow soldering is the most widely used technique in the assembly of surface-mount electronic components. In the reflow soldering process, components are mounted on the corresponding trace area of a circuit board with a solder paste previously printed on the circuit board. Such formed soldering parts are then loaded into a reflow furnace, passing through heating and cooling zones and solder joints between component leads and solder lands on the circuit board are formed by melting, wetting, and solidifying the solder paste. To ensure a good wetting of the molten solder on the joining surfaces, organic fluxes are normally contained in the solder pastes to remove initial surface oxides on both solder and base metal and to keep the surfaces in a clean state before solidification. The fluxes are mostly evaporated into vapor phase during soldering, however, the flux volatiles may cause problems, such as forming voids in the solder joints and contaminating the reflow furnace. After soldering, there are always some flux residues left on the circuit board that may cause corrosion and electric shorts.
Wave soldering, on the other hand, is a traditionally used soldering method for assembling insertion-mount components. It also can be used for surface-mount components by temporarily bonding the components on the circuit board by an adhesive before soldering. For both cases, the circuit boards with components inserted or temporarily bonded have to be cleaned by using a liquid flux to remove oxides on the component leads and solder lands, and then pass through a high temperature molten solder bath. The molten solder automatically wets the metal surfaces to be soldered and solder joints are thus formed. The molten solder in the bath has a high tendency to be oxidized, forming solder dross. Therefore the surface of the solder bath has to be frequently cleaned by mechanically removing the dross, which increases the operation cost and the consumption of the solder. After soldering, flux residues remain on the circuit boards, which brings the same problems as described for reflow soldering.
To remove the flux residues, a post-cleaning process has to be used. Chlorofluorocarbons (CFCs) were normally used as the cleaning agents, but they are believed to be damaging the earth""s protective ozone layer and their use was banned. Although no-clean fluxes have been developed by using a small amount of activators to reduce residues, there is a trade off between the gain and loss in the amount of flux residues and the activity of the fluxes.
A good solution to all the problems described above, including flux volatiles, flux residues, and dross formation, is using a reducing gas as a soldering environment to replace organic fluxes for removing metal oxides. Such soldering technique is called xe2x80x9cfluxless solderingxe2x80x9d. Among various fluxless soldering methods, the use of hydrogen as a reactive gas to reduce oxides on base metals and solders is especially attractive because it is a very clean process (the only by-product is water which can be easily ventilated out of the furnace), and it can be compatible with an open and continued soldering production line (H2 is non-toxic and has a flammable range of 4 to 75%). Therefore, hydrogen fluxless soldering has been a technical goal for a long time.
However, the major limitation of hydrogen fluxless soldering is the inefficient and slow reduction rate of metal oxides in hydrogen at the normal soldering temperature range, especially for solder oxides, which have higher metal-oxygen bond strengths than that of the oxides on the base metals to be soldered. This inefficiency of hydrogen is attributed to the lack of reactivity of the hydrogen molecule at low temperatures. Highly reactive radicals, such as mono-atomic hydrogen, form at temperatures much higher than the normal soldering temperature range. For example, the effective temperature range for pure H2 to reduce tin oxides on a tin-based solder is above 350xc2x0 C. Such high temperatures may either damage integrated circuit (IC) chips or cause reliability problems. Therefore, a catalytic method to assist generating highly reactive H2 radicals in the normal soldering temperature range has been sought by the industry.
Fluxless (dry) soldering has been performed in the prior art using several techniques:
Chemically active halogen-containing gases, such as CF4Cl2, CF4 and SF6 can be used to remove surface oxides for soldering. However, such gases leave halide residues, which reduce solder bond strength and promote corrosion. Such compounds also present safety and environmental disposal problems, and can chemically attack soldering equipment.
Metal oxides can be ablated, or heated to their vaporization temperatures using lasers. Such processes are typically performed under inert or reducing atmospheres to prevent re-oxidation by the released contaminants. However, the melting or boiling points of the oxide and base metal can be similar, and it is not desirable to melt or vaporize the base metal. Therefore, such laser processes are difficult to implement. Lasers are also typically expensive and inefficient to operate, and must have a direct line of sight to the oxide layer. These factors limit the usefulness of laser techniques for most soldering applications.
Surface oxides can be chemically reduced (e.g., to H2O) through exposure to reactive gases (e.g., H2) at elevated temperatures. A mixture containing 5% or greater reducing gas in an inert carrier (e.g., N2) is typically used. The reaction products (e.g., H2O) are then released from the surface by desorption at the elevated temperature, and carried away in the gas flow field. Typical process temperatures must exceed 350xc2x0 C. However, this process can be slow and ineffective, even at elevated temperatures.
The speed and effectiveness of the reduction process can be increased using more active reducing species. Such active species can be produced using conventional plasma techniques.
Gas plasmas at audio, radio, or microwave frequencies can be used to produce reactive radicals for surface de-oxidation. In such processes, high intensity electromagnetic radiation is used to ionize and dissociate H2, O2, SF6, or other species, including fluorine-containing compounds, into highly reactive radicals. Surface treatment can be performed at temperatures below 300xc2x0 C. However, in order to obtain optimum conditions for plasma formation, such processes are typically performed under vacuum conditions. Vacuum operations require expensive equipment and must be performed as a slow, batch process, rather than a faster, continuous process. Also, plasmas are typically dispersed diffusely within the process chamber, and are difficult to direct at a specific substrate area. Therefore, the reactive species cannot be efficiently utilized in the process. Plasmas can also cause damage to process chambers through a sputtering process, and can produce an accumulation of space charge on dielectric surfaces, leading to possible micro-circuit damage. Microwaves themselves can also cause micro-circuit damage, and substrate temperature may be difficult to control during treatment. Plasmas can also release potentially dangerous ultraviolet light. Such processes also require expensive electrical equipment and consume considerable power, thereby reducing their overall cost effectiveness.
U.S. Pat. No. 5,409,543 discloses a process for producing a reactive hydrogen species using thermionic (hot filament) emission of electrons. The energized hydrogen chemically reduces the substrate surface. The thermionic electrons are emitted from refractory metal filaments held at temperatures from 500xc2x0 C. to 2200xc2x0 C. Electrically biased grids are used to deflect or capture excess free electrons. The reactive species are produced from mixtures containing 2% to 100% hydrogen in an inert carrier gas.
U.S. Pat. No. 6,203,637 also disclosed a process for activating hydrogen using the discharge from a thermionic cathode. In this case the emission process is performed in a separate (remote) chamber containing a heated filament. Ions and activated neutrals flow into the treatment chamber to chemically reduce the oxidized metal surface. However, such hot cathode processes require vacuum conditions for optimum effectiveness and filament life. Vacuum operations require expensive equipment, which must be incorporated into soldering conveyor belt systems, thereby reducing their overall cost effectiveness.
Potier, et al., xe2x80x9cFluxless Soldering Under Activated Atmosphere at Ambient Pressurexe2x80x9d, Surface Mount International Conference, 1995, San Jose, Calif., and U.S. Pat. Nos. 6,146,503, U.S. 6,089,445, U.S. 6,021,940, U.S. 6,007,637, U.S. 5,941,448, U.S. 5,858,312 and U.S. 5,722,581 describe a process for producing activated H2 (or other reducing gases, such as CH4 or NH3) using electrical discharge. The reducing gas is present at xe2x80x9cpercent levelsxe2x80x9d in an inert carrier gas (N2). The discharge is produced using an alternating voltage source of xe2x80x9cseveral kilovoltsxe2x80x9d. Electrons emitted from electrodes in a remote chamber produce charged and neutral hydrogen radicals, which are then flowed to the substrate. The resulting process reduces oxides on the base metal to be soldered at temperatures near 150xc2x0 C. However, such remote discharge chambers require significant equipment costs, and are not easily retrofitted to existing soldering conveyor belt systems. In addition, the process is not designed for removing solder oxides.
U.S. Pat. No. 5,433,820 describes a surface treatment process using electrical discharge or plasma at atmospheric pressure from a high voltage (1 kV to 50 kV) electrode. The electrode is placed in the proximity of the substrate rather than in a remote chamber. The free electrons emitted from the electrodes produce reactive hydrogen radicals, a plasma containing atomic hydrogen, which then pass through openings in a dielectric shield placed over the oxidized substrate. The dielectric shield concentrates the active hydrogen onto those specific surface locations requiring deoxidation. However, such dielectric shields can accumulate surface charge that may alter the electric field and inhibit precise process control. The described process is only used to flux base metal surfaces.
Additional prior art of interest include: U.S. Pat. Nos. 3,742,213; U.S. 5,105,761; U.S. 5,807,614; U.S. 5,807,615; U.S. 5,928,527; U.S. 5,985,378; U.S. 6,004,631; U.S. 6,037,241; U.S. 6,174,500; U.S. 6,193,135; U.S. 6,194,036; U.S. 6,196,446; Koopman, et. al., Fluxless Flip Chip Solder Joining, NEPCON WEST ""95 PROCEEDINGS, pp 919-931; and Shiloh, et. al., Flux-free Soldering, NEPCON WEST ""94 PROCEEDINGS, pp 251-273.
The shortcomings of the prior art in providing an economical and efficient fluxless soldering process to remove base metal and solder oxides for superior soldering without oxide or flux flaws in the solder joint are overcome by the present invention, which provides fluxless soldering at low temperatures, near ambient or atmospheric conditions and with low DC power requirements or similar low energy electron propagation to use negatively charged ionic hydrogen to actively react with oxides before or during the soldering operation, as set forth in greater detail below.
The present invention is a method of dry fluxing metal surfaces of one or more components to be soldered, comprising the steps of: a) providing one or more components to be soldered which are connected to a first electrode as a target assembly; b) providing a second electrode adjacent the target assembly; c) providing a gas mixture comprising a reducing gas between the first and second electrodes; d) providing a direct current (DC) voltage to the first and second electrodes and donating electrons to the reducing gas to form negatively charged ionic reducing gas; e) contacting the target assembly with the negatively charged ionic reducing gas and reducing oxides on the target assembly.
Preferably, the reducing gas is hydrogen in an inert gas such as nitrogen to comprise the gas mixture. The components are preferably printed circuit boards or electronic devices, more preferably electronic devices mounted on a printed circuit board. Preferably, as or after the dry fluxing to remove oxides the components are soldered by reflow or wave soldering at elevated temperature.